Controls on the stable isotope composition of daily precipitation in Sydney Australia: 9 years of daily data, including Radon-222

Abstract

Daily and weekly rainfall stable isotopic data, collected over a 9-year period (for daily) and 14-year period (for weekly) at Lucas Heights, Australia, were compared. Small differences in the local meteoric water lines (LMWLs) were seen when developed using daily, weekly, or monthly data (where the effect of short-term small atmospheric variability is reduced by the associated averaging). Although, if a precipitation weighted least squares method was used to derive the LMWLs, the differences between the daily and weekly derived LMWLs were lower.A systematic analysis was then conducted of the impact on the isotopic composition of precipitation from environmental conditions at the sampling site and along the air mass trajectory, the synoptic weather systems, and finally large-scale climate drivers (e.g.,El Niño-Southern Oscillation).The best correlation of δ18O was with local temperature, rainout of the air mass, and rainfall amount at the site. The correlation between precipitation amount and δ18O values was higher when daily samples were used, than when the precipitation weighted monthly values were considered (-0.36 as opposed to −0.31). A good correlation was also seen between δ18O and duration of the precipitation event (r = -0.42). The correlation between δ18O and the fraction of convective rainfall was significant, but low (r = 0.11).On a synoptic systems basis, the lowest δ values were seen in rainfall from offshore low-pressure systems, located to the east or north-east of the site; although, the largest rainfall was seen from low pressure systems arriving from the north-east. The influence of the large-scale climate drivers on the isotopic composition of precipitation was insignificant at this coastal site.When Radon-222 was used to categorise the atmospheric mixing state, rainfall amount was clearly negatively correlated with nocturnal stability, with the lowest amount of rainfall occurring on days with the most stable nocturnal periods. Most of the convective rainfall occurred on these same days, occurring after the warmer period of the day, although the total rainfall amount was low. Analysing the data within the separate radon-derived stability categories improved the correlations with rainfall amount for some of the stability categories.